Generally we get this error when backup device is on the network and we don't have sufficient privileges.
To enable backups to network drives, perform the following steps:
1. The SQL Server service must be started using a domain user account to access any resources on a remote computer. Verify that the MSSQLServer service is started under a domain account that has write access to both the Server share and its underlying partition.
2. Right click on “My Computer” and select “Manage”.
3. Expand “Services and applications” and click on “services”.
4. Select the MSSQLServer service and then right click-> properties -> Log on.
5. Examine the startup options for the service and verify that This Account is selected and that a valid domain account is supplied in the form Domain_Name\Domain_account (with the correct password).
NOTE: If you changed the service to run under a domain account, you must stop and restart the MSSQLServer service.
6. Verify that the account specified has write access on the share to which you are backing up, as well as the underlying partition.To do this, perform the following steps:
a. Right-click the share folder in Explorer.
b. Click Sharing and security on the shortcut menu.
c. On the Security tab, click Permissions.